> From:         M. Adam Davis[SMTP:adampic@UBASICS.COM]
> Sent:         Monday, May 03, 2004 10:29 AM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: [EE:] Smartest binary way to detect current or no current in a cable

> Several years ago a practical electronics article was written up for
> just this type of project.  The author wanted to know when his pipe
> heating cables were on (the cables have temperature sensors built in) so
> he could just check every so often and take action if they seemed to be
> off for too long.

> He wrapped several (many?) layers of wire around one conductor, then
> hooked the coil to an LED and resister.  Since the current didn't vary
> when it was on then the voltage on the coil is known and he didn't need
> any voltage limiting circutry.  If your current varies then you may
> want, at least, a zener diode and a regular diode (to prevent reverse
> overvoltage from breaking the LED).

        How did he get that to work? Wrapping wire around one conductor
      does not make a transformer. The "secondary" wire is at right angles
      to the "primary"; there is no coupling between them.

> You can replace the LED with any optocoupler, and most optocouplers will
> have a higher reverse breakdown voltage rating so you wouldn't need the
> additional diode.  Since you can get an optocoupler with many kinds of
> output it's easy to interface to practically any other project.   A
> simple transister opto and resistor can be used to connect to a
> microcontroller pin.  You could also skip the optocoupler and simply
> rectify and filter the output of the coil and measure that.

> There have also been many projects in the days of yore which switch AC
> loads based on whether another circuit was turned on.  Typically used to
> power printers, monitors, etc when the computer switches on.  The idea
> is the same, so if you can find one then examine the current sensing
> circuit for more ideas.

> -Adam

I used to have a small device which had a line cord coming out one end
and two outlets at the other end. I plugged my stereo turntable into one
of the outlets, and my amplifier into the other. Whenever the turntable
was turning, the amp was on. When the turntable shut off at the end
of the record, the amp turned off. Inside the box was a relay; its coil was
connected in series with the turntable and its contacts powered the amp.
The relay coil had a very low resistance; the relay may have been a low
voltage (AC) type or it may have been specially designed for this device.

John Power

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